FINDLAY — Shares of Cooper Tire & Rubber Co. continued to tumble Monday as the agreement to sell the company to Apollo Tyres Ltd. appears to be in peril.

Cooper on Friday filed a complaint in Delaware Chancery Court, saying that India-based Apollo has been dragging its feet, purposefully preventing the merger from closing. Cooper asked the court to require that Apollo expeditiously close on the $2.5 billion deal.

Apollo, meanwhile, is seeking to lower the previously agreed to and approved $35 per-share transaction price.

Both sides say they remain committed to the deal and believe the merger brings with it significant strategic benefits, but all the turmoil has made investors nervous.

Cooper opened the day Friday at $31.24 per share. It has since lost nearly 18 percent of its value, closing a very heavy trading day Monday at $25.72 per share.

Apollo has yet to respond in court to Cooper’s complaint.

However, in a statement released Sunday, Apollo said between labor issues in the United States, and uncertainty about the status and financials of a Cooper subsidiary joint-venture plant in China, costs of the merger have risen well beyond what was expected and outlined under the merger agreement. The Chinese plant has been hit by a work stoppage.

“Cooper has acknowledged to Apollo that some price reduction is warranted,” Apollo said. “The issue now is by how much.”

An Apollo spokesman declined to say Monday what price the company now believes is fair to pay to acquire Cooper. The spokesman said the company had no additional comment.

In a statement released Monday, Cooper rejected the claim that it is willing to lower its per-share asking price.

“That makes me nervous because the parties are confusing backroom negotiations with the positions they’re willing to take publicly,” said Geoffrey Rapp, a University of Toledo law professor who specializes in business law.

Mr. Rapp said it isn’t out of the ordinary for companies in the midst of a merger to end up in litigation. That said, the nature of Cooper’s complaint could be a bad sign for the deal.

“There’s two ways we could look at this,” he said. “The scary way is that this deal is falling apart and this lawsuit is an effort to [position Cooper] for what is going to be a big nasty fight afterward about what went wrong, whose fault it is, and who should pay for it.”

Mr. Rapp said it’s entirely possible Apollo is worried that it overbid or is bending to pressure from its shareholders, many of whom believe the company is taking on too much debt to finance the deal.

In its Sunday statement, Apollo said it continues to have committed financing from four major international banks.

The merger was announced in June, with both parties saying they expected the deal to close by the end of the year. Cooper’s shareholders voted just last week to approve the sale.

Still, even before last week it hadn’t been completely smooth sailing.

In mid-September, an arbitrator ruled that Cooper’s contracts with the United Steelworkers in Findlay and Texarkana, Ark., included a successorship clause that would require Apollo to enter into an agreement with them.

It isn’t clear exactly what the union is asking for, though Apollo said over the weekend that it is willing to make concessions to reach an agreement — something Cooper wouldn’t do. The company argues that is taking extra time.

“Apollo finds it implausible that Cooper, having failed to resolve these issues for several months, would realistically expect to force Apollo to concede material issues on Cooper’s accelerated timeline,” the company said.